Long-period fiber grating devices provide wavelength-dependent loss. Such gratings couple optical power between two copropagating modes with very low back reflection. They typically comprise a length of optical fiber wherein a plurality of refractive index perturbations are spaced along the fiber by a periodic distance .LAMBDA.. In contrast with conventional Bragg gratings, long-period gratings utilize a periodic spacing .LAMBDA. which is typically at least 10 times larger than the transmitted wavelength .lambda., i.e. .LAMBDA..gtoreq.10.lambda.. Typically .LAMBDA. is in the range 15-1500 micrometers, and the width w of a perturbation is in the range 1/5.LAMBDA. to 4/5.LAMBDA.. In some applications, such as chirped gratings, .LAMBDA. can vary along the length of the grating.
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with conventional Bragg gratings which reflect light, long-period gratings remove light without reflection by converting it from a guided mode to a non-guided mode or by converting it from one guided mode to another. The spacing .LAMBDA. of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength .lambda..sub.p from a guided mode into a nonguided mode, thereby reducing in intensity a band of light centered about .lambda..sub.p. Alternatively, .LAMBDA. can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode).
Long-period grating devices are thus useful as filtering and spectral shaping devices in a variety of optical communications applications. Key applications include spectral shaping for high-power broadband light sources (C. W. Hodgson et al., 9 Optical Society of America Technical Digest Series, Paper TuG3 (1996)), gain equalization for optical amplifiers (A. M. Vengsarkar et al., 21 Optics Letters 336, (1996)), band rejection in cascaded high-power Raman lasers (S. G. Grubb et al., Laser Focus World, p. 127 (February 1996)), and filtering amplified spontaneous emission in erbium doped amplifiers (A. M. Vengsarkar et al., 14 J. Lightwave Technol. 58 (1996)).
While a variety of methods have been used to make long period gratings, most such methods require specially doped fibers and expensive equipment such as ultraviolet lasers. The most popular technique involves fabricating fibers doped to exhibit ultraviolet light photosensitivity and exposing the fiber at periodic regions along its length to high intensity ultraviolet light. See Hill et al., U.S. Pat. No. 5,131,069, Hill et al., U.S. Pat. No. 5,216,739 and Vengsarkar, U.S. Pat. No. 5,430,817. The method can be enhanced by heat, and alternatively, gratings can be written by heat in combination with hydrogen sensitization. See Lemaire et al., U.S. Pat. No. 5,478,371 and Atkins et al., U.S. Pat. No. 5,500,031, respectively.
Another method described by Poole et al. in U.S. Pat. No. 5,411,566 uses a two step process. In the first step a focused CO.sub.2 laser is used to make periodic grooves on the fiber surface by ablation, and in the second step the ablated fiber is annealed so that the material imbalance created by the periodic ablations is transferred to the core-cladding interface. With this process, annealing the fiber in the regions of the cuts transforms the corrugations on the fiber surface to permanent deformations of the core. See Poole, col. 2, lines 53-58 and col. 4, lines 26-27. The result is a periodic change in the effective index of a mode traveling in the fiber.
Despite these various methods, there remains a need for a simpler, less expensive method for making long period fiber gratings.